Production of available phosphate



Patented Mar. 30, I926. v I I UNITED STATES PATENT OFFICE.

HERBERT E. MEYEBS, OF PITTSBURGH, PENNSYLVANIA, ASSIGNOR TO ARMOUR FIR-TIIIIZER WORKS, OF CHIOAGIO ILLDIOIS, A CORPORATION 01 NEW JERSEY.

PRODUCTION 01" AVAIL ABLE PHOSPHATE.

No Drawing.

' To all whom it may concern:

Be it known that I, HERBERT H. Marries, a citizen of the United States,residing at Pittsburgh, in the county of Allegheny and State ofPennsylvania, have invented certain' new and useful Improvements in--theProduction of Available Phosphate, of

which the following is a specification.

My invention relates to a new process or method for the production ofso-called' available phosphate from phosphate rock whereby it may beused to advantage for fertilizing purposes.

So far as the benefits of the new process are reflected in the product,it may be mentioned that the latter is not an acid phosphate, and infact may be slightly alkaline;

the employment of the process resulting 1n a higher grade or greaterpercentage of available phosphate than accrues in the, production ofacid phosphate.

The product of the new method can be mixed with all types of fertilizerswithout danger of having its phosphate revert and without injuring thequality of such other fertilizers.

The phosphate resulting from such improved process is cheaper to makethan acid phosphate, and, because of its lack of madeto render thephosphate of phosphate rock available for fertilizer purposes by variousfire or heat treatments, but up to the present time, however, so far asI am advised, none of these methods have met with commercial success,but rather they have failed, either because the conversion to availablephosphate was not complete enough, or because too much reagent or fuelwas required to produce the desired results economically.

The general method prescribedin such cases is to mix a certain amount ofalkali metal salt with the phosphate rock, either with or without theaddition of carbon or carbonaceous materials, and water, and thenApplication glad October 20, 1922. Serial No. 595,825.

to subject the mixture to tem eratures between 1400 C. and 1500 usuallyin a rotary kiln, such as is commonly employed in Portland cementmanufacture.

The alkali metal salts usually employed are either nitre'cake whichapproaches in composition sodium bisulphate (NaHSO,), or salt cake(lITa,SO,), the amount prescribed varying considerably with a mini-.

mum of about fifteen per cent of the mixture. i

When this procedure is followed on a com mercial scale, the results aredisappointing since the material is either discharged from the kiln in anodular form or it reacts with the lining of the furnace, building uprings of fused material. v

Thenodules are usuallyin a. semi-fused condition, their surfaces beingoften rather porous and thoroughly converted, but their interiorsconsist of strata of fused material.

containing a high percentage of the original reagents and a very lowpercentage of available phosphate.

Reaction with the refractories is of course fatal to high percentages ofconversion to available phosphate.

The chemicalreactlons involved in such known process w re notunderstood, but

since the products showing high conversions usually containedapproximately the same total content of pentoxide (P 0 as the raw rock,it was assumed that, in the course of the treatment, the reagents weredriven off by the heat and did, not enter into the final constitution ofthe product.

In order to develo such method into a practicable, commercial process,whereby a maximum'of conversion could be obtained with a minimum ofreagent, fuel and time,

which was the aim of this invention, it was" necessary to study thechemical reactions involved, in order to arrive at a rational basis fordetermining the amout of reagents required, and to devise practicalmethods of -overcoming the operating. difliculties in the process, suchas the tendency to fuse and to react with the refractories.

Phosphate rocks, in general, contain, besides calcium phosphates,fluorides carbonates, silicaand silicates, iron and a uminum compounds,etc. It was demonstrated that when phosphate rock was heated to asuflicient temperature for a period of time,

fluorine was evolved and oxides and silicophosphates were formed in themass which were somewhat soluble in the oflicial ammonium citratesolution.

For instance, if a. Tennessee brown rock containing approximately 10%silica and 3.5% fluorine is heated to about 1400 C. for an hour, nearly50% of the'phosphate will be rendered available by the ofiicial citratemethod, and silica can be detected in the citrate solution; while thefluorine contents will be reduced to only a few tenths of a percent andthe silicareduced correspondingly.

If a sufficient amount of an alkali metal compond be added to the rawrock in a form Which-reacts at high temperatures as if it were anoxide(M 0) and the-mixture heatedfor a period of time to about 1400 0., 1t

will be found that between 85% and 95% of the phosphate is renderedsoluble or available according to the official citrate method.

Moreover, if a proper amount of alkali metal salt has been used, it willbe found that the product contains only traces of the acid radical whichthe alkali metal salt. had

' contained, but a definite amount of alkali metal oxide combined insome manner with the other constituents of the product.

Further, it will be discovered that the citrate solution containsdissolved in it besides silica some of the alkali metal oxide.

In other words, the indications are that a I double silico-phosphate oflime and alkali rate solution.-

metaloxidehas been formed which is comparatively readily soluble in theoflicial cit- The amount'of alkali bears a; definite relation to thecomposition of the raw rock, and, therefore, a minimum theoreticalrequirement of alkali metal salt canvbe calculated for any givencomposition is in the neighborhood of 1000 undermost conditions, itwould not prove rock.

For practicaloperation, however, it is not always expedient to usemerely this minimum quantity of reagent, because of they effect upon thephysical condition of the product, and on the time and temperature reuired for the com letion of the reaction. xperiment has shown that thereis a definite relation between the three factors amount of reagent,length of time of heating, and temperature for the reaction, and aneconomic-balance can be struck among these factors to meet a given setof conditions.

The minimum temperature at which the reaction apparently-takes placeeffectively 0., but,

economical to operate at this temperature as the time requiredtocomplete the reaction and the amount of reagent necessary are too- Inshort, a major portion, at least, of the reagent is decomposed and itsacid radical is evolved as a gas while its alkaline radical metal'oxiderequired:

enters v the rock.'

As long as the gases are evolved and swept away by a draft thrbfigh thefurnace, the reaction continues as the equilibrium is continuouslydisplaced in favor of the further decomposition of the reagent, but anycondition which interferes with the free cvolution of gas and itsremoval from the sphere of reaction tends to retard the reaction and topermit an equilibrium being reached Beyond which there is no furtherdecomposiion.

Local fusion, such as occurs in the nodules and clinkers produced by theusual process, causes a pressure in the interior of the nodules whichhinders the free evolution of gas with the result that, while thereaction is fairly complete on the surface of the clinkers, in theirinteriors it is incomplete and there is a large percentage ofundecomposed rea ent.

urther, if the reaction is carried out in a closed receptacle instead ofone through which a free draft passes, the reaction is very incompletesince the evolved gases are not removed and equilibrium is soonattained. Y

It will be apparent, therefore, that an initial porous or cellularstructure of the evolution of gases and an easy penetration of thedraft, is very desirable, and such a condition also assures a mostintimate contact between the rock and reagent which is requisite forcomplete reaction since, in order to react, physical t icles isnecessary.

f In the usaul processes the charge is introduced intov the furnace inthe form of a powdered mass, and it is quite possible that a certainamount of segregation occurs be.-

tween the rock and reagent in its travel down the kiln. V

Moreover, this method of introduction offers an excellent opportunityfor reaction ihto-reacthih with the constituents of charge which willpermit of a ready and free contact between the parbetween the charge andthe refractory furnace lining, the product.

At the temperatures used in the furnaces,

the phosphates and alkali salts are .very rewhich ruins both the liningand active and will attack the furnace lining) '0 100 C. to 200 C. lowerthan when a charge 2nd,: A greater ease of evolution of gases.

3rd: The avoidance of local fusion. 4th:' The less effect upon therefractories, and 4 v 5th: The lower temperature of reaction.

In practice, this porous or cellular structure is attained by sinteringthe charge before it is introduced into the reaction furnace, and,although this sintering may he performed in a number. of ways. the

' preferred process is the Dwight and Lloyd method, in accordance withwhich the charge is intimately mixed with fuel and slightly moistened,spread in a uniform thickness upon a horizontal moving hearth, and thefuel ignited, the-combustion being then-sustained in the mass by adowndraft through it. At the end of thetravel of the hearth the fuelwill have been consumed and the charge will be sintered together in avery desirable cellular condition, the temperatures attained in the masson the hearth being controlled by the amount of fuel and draft.

In carrying out this new and improved process with the phosphate rockand reagent, it is usual to attain such tempera-- tures on the hearth ofthe Dwight and Lloyd machine that the reaction between them is partiallycompleted and the sintered material, therefore, contains availablephosphates up to of the total phosphate, a greater conversion not takingplace since the time at which the mass is at the optimum temperature istoo short.

The method of treatment of :the sinter which is usually preferred, is tocrush it to a uniform size of such dimensions that it will be heatedthrough uniformly, and then to calcine it at the proper temperatureforthe required time in a rotary kiln or multiple hearth furnace.

The details of such preferred method are as follows:

If a Tennessee brown rock containing approximately 31.5 percent P 0 and10 percent silica is to be treated, there will be required an amount ofreagent containing approximately 9.2 percent of the weight of the rawrock as Na,O if the calcining treatment is to be at about 1250 C. for 30minutes or 1400 C. for 20 minutes.

' factory.

' I This is equivalent to about 30 percent of the weight of the raw rockin the form of the average nitre cake or 21- percent of salt cake, thesebeing probably the cheapest salts for usein this process.v v

7 Therefore 100 parts of the rock and -30 parts of nitre cake or 21parts of salt cake are ground together intimately in'a suitable typeof'mill and of course the finer the grinding the more intimate themixing and the better the results. For practical purposes 95% through100 mesh is adequate and even coarsermaterial than this has provensatis- This charge is then mixed, according to the standard methodsemployed in the Dwight and Lloyd *sinterin practice, with fuel which ispreferably eit er coke breeze or anthracite buckwheat,-enough fuel beingused so that the charge contains about" 9% fixed carbon. Sufiicientwater is added with the fuel to cause the particles of-the charge toadhere to each other, and the charge is then sintered under suchconditions that the time between ignition and completion of thesintering is about 25 to 30 minutes.

The sinter as it is discharged from the Dwight and Lloyd machine iscrushed to about inch to 1 inch size and introduced into a rotary kilnof the usual type. employed in cement practice where it is graduallyheated to the reaction temperatures and the hot zone so extended thatthe desired tem perature-is attained for the required length of time, inthe case being discussed, 1400 C. or more for 20 minutes, which is acommon condition in cement practice, or 125 0 C. for 30 minutes. Thelatter is a temperature somewhat lower than that employed in ordinarycement practice and it requires. a slightly longer hot zone than isprobably common.

The calcined material is cooled after it is discharged and is thenground to 95% through mesh. It will contain approxi- 'mately 31.5% totalP 0 of which is available by the citrate method. The finenessof grindingthe finished product influences the solubility in citrate, but .forpractical purposes the above fineness is sufcient.'

Those skilled. in this art will readilysintering a mixture containingground phosphate rock and an alkali metal salt thereby effecting apartial conversion of the phosphate, crushing such sintered mixture intoporous lumps, and calcining such lumps at asuflicient temperature andfor a period of time to complete. the conversion of the phosphate intoavailable phosphate.

2. The processof rendering the phosphate of phosphate rock available,consisting in sintering a mixture containing ground phosphate rock, analkili metal salt, and carbo- Jaceous'matcrial thereby effecting apartial conversion of the phosphate, crushing such sinte red mixtureinto porous lumps, and calcining such lumps at a suflicient temperatureand for a period of time to complete She conversion of the ablephosphate. 7 1 I 3. The process of rendering the phosphate of phosphaterock available, consisting in phosphate into availsintering a mixturecontaining ground phosa period oftime sufiicient to complete theconversion of the phosphate to available phosphate.

4. The process of rendering the phosphate of phosphate rock available,consisting in grinding together about 100 partsof phosphate rock andapproximately 30 parts of nitre cake so that about 95% will pass through100 mesh, mixing carbonaceous material with such charge so that thelatter will contain approximately 9% fixed carbon, adding sufiic-ientwater to cause the particles to adhere to each other, sintering suchcharge under such conditions that the time between i nition andcompletion of the sintering is a out to minutes, crushing thesinteredimaterial to about to 1 inch size, calcining such crushedsintered material at a temperatureof about 1250 C. to 1400 C. for aperiod of approximately 20 minutes to 30 minutes, and grinding thecalcined material so that about 95% thereof will pass through a screenof 80 mesh.

In witness whereof I have my hand and seal. l

HERBERT H. MEYERS. a s.]

hereunto set-

